This invention relates to four-stroke, internal combustion engines, particularly lightweight, four-stroke engines for operator-carried power tools.
It is conventional practice in the design of outdoor power tools to use a two-stroke internal combustion engine or an electric motor for powering an implement such as a line trimmer, a blower/vacuum or a chain saw. Two-stroke internal combustion engines are relatively light and may readily be carried by an operator during operation with various angular orientations. Two-stoke engines, however, have well-recognized exhaust emissions problems that often make them unfeasible for their use in areas that must comply with exhaust gas emissions regulations such as the California Air Resource Board (CARB) and Federal EPA regulations dealing with California air quality.
Limitations on the exhaust emissions of carbon monoxide, hydrocarbons and oxides of nitrogen that will be required in the year 2000 cannot feasibly be met by outdoor power tools powered by using two-stroke internal combustion engines. Four-stroke internal combustion engines, on the other hand, provide a distinct advantage for outdoor power tool manufacturers in their attempt to meet the 2000 CARB emissions regulations. In addition, they operate quieter compared to two-stroke engines.
Unlike two-stroke engines which simultaneously admit a fresh charge of fuel and air mixed with lubrication oil while exhausting combustion products, including unburned fuel, a four-stroke internal combustion engine maintains the lubricating oil relatively isolated from the combustion chamber.
The intake and exhaust valve region and the camshaft and cam drive portions of a four-stroke internal combustion engine are lubricated with oil supplied from the crankcase. The combustion chamber remains relatively isolated from the crankcase. Lubricating oil is not introduced into the air/fuel mixture as in the case of a two-stroke engine. Lubrication of the cylinder wall occurs as a film of crankcase oil develops on the cylinder wall. The piston rings seal the variable volume combustion chamber and effectively prevent mixing of the lubrication oil with the combustion gases.
One disadvantage of using four-stroke internal combustion engines for outdoor power tools, aside from a weight disadvantage, traditionally has been the inability to operate the engine upside down or at the extreme tilt angles that would be required by the operator. Oil in the crankcase in those instances would tend to be drained through the engine block to the intake and exhaust valve region of the engine and would enter the air/fuel mixture intake flow path as well, thereby upsetting otherwise efficient fuel combustion during operation. Continuous operation of the engine, even for relatively short periods, may result in piston seizure because of an interruption of lubrication oil flow to the cylinder wall.
Many of the shortcomings associated with the use of four-stroke engines with outdoor power tools have been solved by the teachings of U.S. Pat. Nos. 5,241,932 and 5,421,292, which are incorporated in the present disclosure by reference. Those patents, which are assigned to the assignee of the present invention, disclose engines which avoid the weight disadvantage of using a four-stroke internal combustion engine in an outdoor power tool. Further, they make provision for efficient engine operation throughout a wide range of angular dispositions or orientations.
Copending U.S. patent application Ser. No. 08/614,835, filed Mar. 8, 1996, which also is assigned to the assignee of the present invention, describes a further improvement in the earlier prior art teachings. That improvement makes it possible to maintain distribution of lubrication oil mist throughout the engine while maintaining the liquid lubrication oil isolated from the valve chamber and the cam and cam drive system. The engine of the copending application, which is incorporated in the present disclosure by reference, includes a lubricating oil mist passage that is formed in the crankshaft and in the crankshaft counterweight. The crankshaft passage facilitates distribution of lubricating oil in the form of an oil mist through the region of the camshaft and the camshaft drive of the engine and through oil mist flow passages to an overhead valve chamber.
A lubricating oil mist generator driven by the crankshaft agitates liquid oil in the crankshaft and develops a lubricating oil mist which is transferred through the engine in a flow path defined in part by the crankshaft passage. Large oil droplets in the oil mist and liquid oil in the crankcase are prevented from entering the crankshaft passage by the effect of centrifugal force due to the rotation of the crankshaft and the crankshaft counterweight during engine operation. A fine oil mist, however, may pass through the crankshaft passage and through the engine lubrication system since the mist is relatively unaffected by the centrifugal forces created by the rotating crankshaft.
The present invention is a further improvement in engines of the type shown in the ""932 and ""292 patents and in the copending patent application previously identified. Unlike the previously disclosed engines, however, the engine of the present invention does not include a breather or crankcase scavenger region at an upper portion of the engine assembly, such as the top of the valve chamber, or at the upper side of the engine block. Rather, the present invention comprises a closed, multiple-region oil mist containment that includes the crankcase, the camshaft and cam gear drive portion of the engine, and the valve chamber. These regions of the engine assembly are in communication, one with respect to the other, by reason of an oil mist flow passage window in the crankcase portion of the engine assembly.
The engine embodiment of this disclosure includes an air/fuel carburetor with an operator-controlled butterfly valve. The improvements of the invention may be used, however, with engines having other types of air/fuel controls (e.g., electronically controlled fuel injectors).
Oil mist in the engine of the present invention is generated by an oil mist generator or splasher that is driven by the crankshaft so that liquid oil in the crankcase readily is converted into a lubricating oil mist. As in the case of the engine of the copending application, the crankshaft of the present invention includes a radial passage in the crankshaft and in the crankshaft counterweight. Unlike the engine of the copending application, however, the crankshaft passage of the engine of the present disclosure distributes crankcase gases through the engine and ultimately to the air/fuel induction passages. The centrifugal force acting on the crankcase gases, which would include the oil mist itself, ensure that the crankcase gases will be relatively lean before they enter the air cleaner and air/fuel intake region of the engine. This contributes to efficient engine operation and makes it possible to achieve the most energy-efficient combustion since the air/fuel mixture determined by the engine carburetor controls will not be adversely affected by the presence of the lean crankcase gases during positive crankcase ventilation.
The intake end of the crankshaft passage of the engine of the present invention is not in direct communication with the liquid oil in the crankcase. This is achieved by using a strategic crankcase geometry which allows the level of the liquid oil in the crankcase to be separated at all times from the crankshaft passage regardless of the angular orientation of the engine. When the engine is not running and the lubricating oil in the crankcase is in liquid form, the liquid oil will be prevented from passing through the positive crankcase ventilation passage structure. Liquid crankcase lubricating oil, therefore, will not enter the intake and exhaust valve area at the top of the engine, nor will it enter the air/fuel intake region of the engine. The positive crankcase ventilation passages and the crankcase scavenger system will be completely isolated from the valve chamber, the cam and cam gear drive region of the engine and the combustion chamber when the engine is shut down.
According to another objective of the present invention, provision is made to physically separate the power cylinder region of the engine from the crankcase region by providing baffles as part of the engine crankcase casting. The baffles extend across the bottom skirt of the power cylinder. A piston rod, which connects the engine piston to the crankshaft, extends through an aperture between the baffles. The wall of the cylinder below the piston during operation of the engine thus is semi-isolated from the crankcase, thereby reducing the amount of oil that is distributed from the crankcase into the power cylinder. Sufficient oil for lubrication purposes is maintained, but excess oil is inhibited by the baffles from entering the cylinder bore. This feature further improves the operating efficiency of the engine and reduces oil consumption.
The cam chamber, which encloses the cam and cam gear drive, is formed in the cylinder block and in the crankcase. A cylinder head secured to the cylinder block defines the intake and exhaust ports and a valve cover secured to the cylinder head defines a valve chamber for intake and exhaust valves. A rocker arm assembly in the valve cover actuates the valves, and push rods drivably connect the cam to the rocker arm assembly. Push rod openings in the cylinder block provide communication between the valve cover and the cam chamber.
Oil mist in the crankcase is distributed to the cam chamber through the oil mist flow passage window.